In fabrication of highly integrated semiconductor devices, microwave-plasma processing apparatuses have been widely used. In such a microwave-plasma processing apparatus, reaction gases and microwaves are introduced into a vacuum chamber to generate gas-discharge, so that a plasma is produced in the chamber. The plasma is applied to a substrate to etch the surface thereof, or to form thin film layers thereon. Especially, in dry-etching technology and embedding technology, microwave-plasma processing apparatuses controlling generation of the plasma and acceleration of ions in the plasma independently have become important and been studied extensively.
FIG. 1 is a cross-sectional view illustrating a microwave-plasma processing apparatus disclosed in Japanese Patent Laying Open, Kokai Heisei 6-104098 filed by the common applicant to this application. The invention aimed to realize control of generation of a plasma and accelyration of ions in the plasma independently. In FIG. 1, a numeral reference "11" represents a reaction vessel, which is made of metal such as stainless steel, aluminum, or the like. The reaction vessel 11 has a periphery wall of double structure forming a coolant path 12. Along the coolant path 12, coolant circulates from a coolant inlet 12a to a coolant outlet 12b. A reaction chamber 13 is formed in the reaction vessel 11. The reaction vessel 11 is sealed at the top with a microwave introducing window 14, which is made of dielectric material, such as quartz glass, Pyrex glass, alumina, etc., having a low dielectric loss and heat resistance. For heating the interior of the reaction chamber 13, an electric heater, not shown in FIG. 1, is provided around the reaction vessel 11. Both by heating with the electric heater and cooling with the coolant circulating in the coolant path 12, the inside the reaction chamber 13 is controlled in temperature precisely.
The microwave introducing window 14 is provided on the lower surface with a conductive plate 31 of metal acting as a grounded electrode. The conductive plate 31 is provided with a plurality of microwave transmission openings 32, arranged perpendicularly to the direction of traveling of microwaves. The conductive plate 31 is grounded (33) through the reaction vessel 11. The conductive plate 31 may be placed in the middle between the microwave introducing window 14 and an object holder 15a, while the conductive plate 31 is grounded (33) through the reaction vessel 11, although the arrangement is not shown.
In the reaction chamber 13, the object holder 15a holding an object 30 to be processed is placed on a stage 15, which moves up and down by a driver (not shown). The object holder 15a is connected to a high-frequency power supply 18 to generate bias voltage on the surface of the object 30. The object holder 15a is provided with a chucking mechanism (not shown), such as an electrostatic chuck to hold the object 30 firmly. The object holder 15a is also provided with a cooling mechanism (not shown) in which coolant circulates for cooling the object 30. On the bottom of the reaction vessel 11, a gas outlet 16 connected to an exhaust apparatus (not shown) is formed. On the side wall of the reaction vessel 11, a gas inlet 17 is formed to introduce predetermined reaction gas into the reaction chamber 13.
Over the reaction vessel 11, a dielectric line 21, composed of a metal plate 21a of aluminum, or the like, and a dielectric layer 21c, is provided. The end of the dielectric line 21 is sealed with a reflecting plate 21b of metal. The dielectric layer 21c is attached on the lower surface of the metal plate 21a. The dielectric layer 21c is made of fluorine resin, polyethylene, polystyrene, or the like, having a low dielectric loss. The dielectric line 21 is connected through a waveguide 22 to a microwave oscillator 23, so that microwaves generated by the microwave oscillator 23 travel through the waveguide 22 into the dielectric line 21.
In the above mentioned microwave-plasma processing apparatus, for etching the surface of the object 30 held by the object holder 15a, the position of the stage 15 is adjusted in height so that the object 30 is placed at the appropriate position. Next, unnecessary gases are discharged from the reaction chamber 13 through the gas outlet 16, then the reaction gas is introduced from the gas inlet 17 into the reaction chamber 13 until the pressure thereof reaches a predetermined level. At the same time, the coolant is introduced from the coolant inlet 12a into the coolant path 12, and is discharged from the coolant outlet 12b. Subsequently, microwaves generated by the microwave oscillator 23 travels along the waveguide 22 to the dielectric line 21. When the microwaves are introduced in the dielectric line 21, electromagnetic field is generated under the dielectric line 21, and the microwaves pass through the microwave transmission openings 32 into the reaction chamber 13. In response to the microwaves, a plasma is generated in the reaction chamber 13. After that, when high-frequency voltage is applied from the high-frequency power supply 18 to the object holder 15a, bias voltage is generated on the surface of the object 30. With the stable bias voltage, ions in the plasma are irradiated vertically to the surface of the object 30 to be etched, while the energy of the ions is controlled.
In the microwave-plasma processing apparatus, the grounded potential is stable relative to the plasma, because the conductive plate 31 with the microwave transmission openings 32 are contacted on the microwave introducing window 14. As a result, the plasma potential becomes stable in the reaction chamber 13, and therefore, the stable bias voltage can be applied to the surface of the object 30 uniformly. Consequently, ion energy in the plasma can be well controlled, and the ions can be irradiated to the surface of the object vertically.
According to the conventional microwave-plasma processing apparatus, however, the microwave introducing window 14 expands with the passage of time, when the apparatus keeps operating for a long time, so that an undesirable gap is formed between the conductive plate 31 and the microwave introducing window 14 because of a difference of thermal coefficient of expansion between them. As a result, undesirable electrical discharge may occur within the gap, and it becomes difficult to generate stable bias voltage on the object 30.
There is another disadvantage in that the microwave introducing window 14 is eroded with plasma, because the microwave introducing window 14 is directly applied with plasma. Especially, the microwave introducing window 14 made of quartz glass, or the like is easily eroded with the halogen series of gas plasma, such as chlorine, fluorine and bromine. As a result, the microwave introducing window 14 is changed in width with the passage of time. Therefore, the density and distribution of plasma changes, and it becomes difficult to carry out plasma processing uniformly and stably.